The early detection of diseases, such as cancer and heart disease, may significantly improve patient survival. For example, present methods of breast cancer detection include screening mammography and palpation, either by patient self-examination or clinical breast exam. Palpation relies on the manual detection of differences in tissue stiffness between breast lesions and normal breast tissue. The success of palpation is due to the fact that the elastic modulus (or Young's modulus) of malignant tumors is often an order of magnitude greater than that of normal breast tissue. That is, cancerous lesions feel “hard” or “stiff” as compared to normal breast tissue. See T. Krouskop et al., Ultrasonic Imaging 20, 260-274 (1998); A. Saravazyan et al., Acoustic Imaging 21, 223-240 (1995).
As another example, atherosclerosis is a medical condition that involves the stiffening of the arteries, which can occur prior to and in addition to the formation of focal lesions. A human artery has three layers. The innermost layer is relatively thin compared to the other two layers and is commonly referred to as the endothelium layer or intimal layer. The middle layer is a smooth muscle layer (media layer), and the outer layer is a connective tissue layer (adventita). Atherosclerosis can be caused by plaque build up between layers in the arteries, which can lead to conditions that include blockages in blood flow, poor circulation, myocardial infarction, aneurisms, and stroke. The development of atherosclerosis can be influenced by factors such as diet, exercise, smoking habits, and other medical conditions such as diabetes. Treatments for atherosclerosis include drug therapy, changes in diet, increased exercise programs, and smoking cessation.
Focal lesions, which protrude into the blood vessel, may be detected by a variety of methods, including cardiac catheterization, intravascular ultrasound, conventional B-mode and Color Doppler ultrasound, and electron beam computed tomography. However, atherosclerosis may occur prior to the formation of focal lesions and does not necessarily involve plaque protrusions that could be detectable using conventional testing methods. Atherosclerosis may also be characterized by increased blood pressure. However, blood pressure levels do not directly correlate to degrees of atherosclerosis, and elevated blood pressure levels may not exist in a subject without atherosclerosis.
U.S. Pat. No. 5,921,928 to Greenleaf uses sound waves to vibrate tissue and monitors the response of the vibrating tissue. A disadvantage of such an approach is that different transmit and detection systems may be required, and multiple pushing cycles at a single location may be required. Moreover, ultrasonic stimulated acoustic emissions may not result in a form feasible for actual clinical diagnosis.